Polyester films can be formed in a continuous manner into large area films which cannot be produced with other resin materials. In addition, because of their good properties in strength, durability, transparency, flexibility and surface characteristics, they have been used in the fields needing them in large quantities, such as magnetic recording, industrial, packaging, agricultural and building materials. Among them, biaxially oriented polyester films have been used in various fields because of their good mechanical, thermal and electric properties and good chemical resistance. In particular, as base films for magnetic tapes, polyester films are unrivaled by other films in usefulness.
In recent years, in the process for processing a polyester film, the process for deposition and application of a magnetic layer during the magnetic recording medium production or the process for application of a heat sensitive transfer layer during the thermal transfer material production, it is demanded to increase the processing speed or to further improve the quality of a finished product. As these requirements grow, it is also required that polyester films have a film surface with more improved running durability and wear resistance.
To meet the requirements stated above, it is known to be effective to uniformly form fine protrusions on a surface of a polyester film. For example, a polyester film is known in which substantially spherical silica particles such as colloidal silica are contained to provide fine protrusions on the film surface (e.g., Japanese Unexamined Patent Application Publication No. 59-171623). A polyester film is also known in which a thin film layer containing fine particles that provide surface protrusions is laminated on a base layer (e.g., Japanese Unexamined Patent Application Publication Nos. 62-130848, 2-77431 and 8-30958).
On the other hand, magnetic recording media are becoming denser with each passing year and the wavelength employed for recording is becoming shorter, and the mode of recording is shifting from analogue form to digital form. When a ferromagnetic metal thin film layer is provided on one surface of a base film to produce a magnetic recording medium, the ferromagnetic metal thin film layer is usually provided on an ultra-flat film surface. In this case, since the ferromagnetic thin film layer generally has a thickness of as thin as about 0.02 to 0.5 μm, the surface geography of the base film may be directly reflected as the surface geography of the finished ferromagnetic thin film. Therefore, it is strongly required to reduce the height of surface protrusions of a base film and form ultra-fine protrusions at high density to provide surface smoothness and surface slipperiness, and development of films having a surface that satisfies these requirements has been demanded.
If ultra-fine particles are contained in the film surface in a large amount, however, generation of coarse protrusions caused by aggregation of the particles cannot be avoided and it would be difficult to form fine protrusions uniformly and at high density by using particles. Although addition of particles is effective for imparting surface slipperiness to the finished film and reducing the friction coefficient between the film and a conveyor roll during the film forming/processing process, problems still remain that coarse protrusions may drop off onto the conveyer roll so as to scratch the film and that the output characteristics of a magnetic tape produced using the film may be deteriorated. Therefore, it is quite difficult to employ the above-mentioned particle addition method as the means for forming ultra-fine protrusions at a high density in the industrial production.
An alternative method is known in which desired fine protrusions are formed on a surface by the action of crystallization of polyester without relying on the particle addition method (e.g., Japanese Unexamined Patent Application Publication No. 7-1575). According to the method utilizing the crystallization of polyester, the finished film can have good running durability and wear resistance since generation of voids around the particles can be prevented.
As the techniques for utilizing the crystallization of polyester, there are known a heat treatment method by winding a polyester film around a heated roll; a heat treatment method with an infrared heater; and a method of heating with a stenter. However, these methods have serious problems of (1) and (2) below. In these conventional heating methods, since the whole film is heated, there is also such a problem that troubles resulting from slack and adhesion of the film may frequently occur.
(1) It is difficult to stably produce high quality polyester films each having identical quality, since the number of fine protrusions would vary depending on apparatus-specific variable factors such as unevenness in temperature during the film formation.
(2) It is impossible to increase the film formation speed, since it takes much time to form ultra-fine protrusions by crystallization of polyester.
Alternatively, it has been employed to irradiate a film with ultraviolet light for modification of chemical properties and patterning of a film surface. For example, irradiation with ultraviolet light is employed in the coating or lamination of a chemical substance (e.g., UV-curable resin) on a surface of a polyester film to improve the adhesion, cohesion, anti-static properties, mechanical properties, optical properties and so on or in the patterning of the surface with a photosensitive resin (e.g., Japanese Unexamined Patent Application Publication No. 11-65130). There is a method in which an ultraviolet curable resin layer containing powder particles is provided on the film surface and the curable resin layer is then irradiated with ultraviolet light to form recessed-and-projecting patterns on the surface (e.g., Japanese Unexamined Patent Application Publication Nos. 11-277451 and 10-296944). As a method for chemically modifying the film surface, there is a method in which active oxygen is generated by the combination of ozone treatment and ultraviolet ray treatment to improve the adhesion of the film surface (e.g., Japanese Unexamined Patent Application Publication Nos. 5-68934 and 11-236460).
As mentioned above, irradiation of a film with ultraviolet light has been employed for denaturing a chemical substance applied on the film (e.g., curing of an ultraviolet curable resin layer) or for modifying the chemical properties of the film surface (e.g., improvement in adhesion of the surface).
The object of the present invention is to provide a high quality polyester film that has a surface superior in running durability and wear resistance, exhibits good output characteristics when used in a magnetic recording medium, and is superior in productivity, process simplification and production cost, and also to provide a production process for the polyester film.